Arsenic trioxide inhibits nuclear receptor function via SEK1/JNK-mediated RXRα phosphorylation

We have previously published that 2 proven treatments for acute promyelocytic leukemia, AS(2)O(3) and retinoic acid, can be antagonistic in vitro. We now report that As2O3 inhibits ligand-induced transcription of the retinoic acid receptor, as well as other nuclear receptors that heterodimerize with the retinoid X receptor alpha (RXR alpha). As2O3 did not inhibit transactivation of the estrogen receptor or the glucocorticoid receptor, which do not heterodimerize with RXR alpha. We further show that As2O3 inhibits expression of several target genes of RXR alpha partners. Phosphorylation of RXR alpha has been reported to inhibit nuclear receptor signaling, and we show by in vivo labeling and phosphoamino acid detection that As2O3 phosphorylated RXR alpha in the N-terminal ABC region exclusively on serine residues. Consistent with our previous data implying a role for JNK in As2O3-induced apoptosis, we show that pharmacologic or genetic inhibition of JNK activation decreased As2O3-induced RXR alpha phosphorylation and blocked the effects of As2O3 on RXR alpha-mediated transcription. A mutational analysis indicated that phosphorylation of a specific serine residue, S32, was primarily responsible for inhibition of RXRa-mediated transcription. These data may provide some insight into the rational development of chemotherapeutic combinations involving As2O3 as well as into molecular mechanisms of arsenic-induced carcinogenesis resulting from environmental exposure.